A steady downpour of rain may cause minor flooding for a large part of Southwest Washington, forecasters said Sunday. The Weather Service issued a flood watch to last until late Monday afternoon. The Cowlitz River and Grays River near Rosburg are expected to near or surpass minor flood stage. Low-lying areas, smaller creeks and streams and poorly draining urban areas also are prone to flooding, according to the Weather Service. The heavy rains, which started Sunday afternoon, are caused by a tropical storm system bringing more warm, moist air into the region, said Gerald Macke, a meteorological technician with the Portland Weather Service. "This is a really juicy storm. Warmer air can hold a lot more moisture than normal (cold air from Alaska)," Macke said Sunday. Macke said rains may last until noon and bring up to 2.5 inches of precipitation to Kelso-Longview. Landslides and debris flows are possible during the flooding, the Weather Service warned. A strong warm front blowing in from the Pacific on Sunday was expected to drench Western Washington and likely trigger avalanches, landslides and floods, including severe flooding at the Tolt River near Carnation. The National Weather Service on Seattle forecast the Tolt's waters to rise above flood level around 10 p.m. on Sunday, cresting around 10 a.m. on Monday and receding back Monday night. "Dangerously deep and quick floodwaters will cover roads and driveways, with some homes flooded," the National Weather Service said in an advisory Sunday afternoon. Flooding will occur all along the river, impacting tributaries and other streams nearby. The agency reported that heavy rain had begun to fall over Western Washington, spreading to the Cascades early Sunday evening. It advised the public to not drive through flooded areas. The agency also issued a flood warning for the Puyallup River near Orting. Puyallup Police in its Twitter feed Sunday warned drivers about "lots of heavy rain tonight and tomorrow," asking them to "use caution and slow down for standing waters."

The Washington State Department of Transportation (WSDOT) reported mixed rain and sleet at Snoqualmie Pass by early Sunday evening. An avalanche warning was in effect for the Cascades near and west of the crest, lasting Sunday night through Monday. "Very dangerous avalanche conditions" were expected to develop as a result of heavy rain and rising snow levels. Earlier on Sunday the National Weather Service had predicted that the warm front could dump 2 to 4 inches of rain on the lowlands and 6 to 15 inches of new snow on the mountains, followed by 2 to 6 more inches of rain in the mountains. The downpour - peaking Sunday night through Monday morning - could overwhelm rivers in Western Washington, where a general flood watch is in effect from 8 p.m. through late Monday night. Landslides also are possible, especially on coastal bluffs and other steep hillsides, and the risk, though diminishing after Monday, will continue for several days after the rain stops. The National Weather Service also advises against travel in avalanche zones and has issued an avalanche watch near and west of the crest of the Washington Cascades through Monday. WSDOT was requiring chains Sunday on all vehicles without all-wheel drive traveling through Snoqualmie Pass on Interstate 90. Traction tires were required for Stevens Pass and advised for the Blewett and White mountain passes. Eastbound I-90 at Snoqualmie Pass was closed briefly at Milepost 47, Denny Creek, early Sunday afternoon because of vehicle spinouts in the slush and snow. The warm front, which occurs when a mass of warm air replaces colder air, is expected to give way to drier, cloudy weather on Tuesday and Wednesday and colder temperatures later in the week.